Many high performance gas turbine engines have a history of marginal afterburner (A/B) ignition performance. Afterburner (A/B) initiation faults have been experienced both on the ground and at altitude, particularly in the upper left hand corner of the engine operating envelope where the low pressure and temperature engine inlet conditions are not conducive for A/B initiation.
A prior system developed by the General Electric Company had a ram air supplied combustor which was swirl stabilized. The primary function was to achieve ignition within the pilot burner can and to subsequently propagate to the full A/B. Thus, the quickness and consistency of A/B ignition is directly dependent on the performance of the pilot burner can. The prior system is described in a paper entitled "Afterburner Ignition Improvement Package" by Thomas Edwards, et al, a copy of which is supplied with the Invention Disclosure Statement filed herewith, said paper being incorporated herein by reference. The present invention is a improvement over the prior system.
The prior invention worked well to eliminate A/B no-lights in the upper left hand corner of the flight envelope, however intermittent problems were experienced at take-off and mid-envelope conditions. These problems are due to an over-rich fuel schedule, which is fixed by this invention.
The prior art fuel schedule controls fuel into the pilot burner by making it directly proportional to compressor discharge pressure of the turbojet engine. Compressor discharge pressure is approximately proportional to A/B pressure. The final metering of fuel into the burner is accomplished within the ignitor fuel tube by a fixed orifice device. For high values of A/B pressure, the fuel injected into the burner potentially could exceed the rich limit resulting in slow or no light due to excessive richness. In accordance with this invention, a variable orifice such as a Lee "Flosert" is provided in conjunction with the fixed orifice to yield acceptable fuel flows at low A/B pressure, and to limit flow to a fixed value at high A/B pressure.
A search of the prior art revealed the following patents:
U.S. Pat. No. 4,128,995 issued to Toot; PA1 U.S. Pat. No. 3,908,363 issued to Bauerfeind; PA1 U.S. Pat. No. 3,760,590 issued to Bader; and PA1 U.S. Pat. No. 3,690,094 issued to McCombs, Jr.
The Toot patent describes a method and apparatus for stabilizing an augmentor system. According to Toot, sensor devices measure temperature and pressures at critical points, and the measured values are compared to reference values. Any difference in the values indicates the proportional amount that the fuel flow is to be biased.
The Bauerfeind patent is directed to an aero gas turbine afterburner control system. The system includes an arrangement in which there is a direct mechanical or electrical connection between the thrust nozzle actuating mechanism and the afterburner fuel control unit. Such an arrangement provides for the degree of afterburning to be controlled as a function of the actual final discharge or exhaust area.
The Bader patent relates to a device for metering ignition fuel to the ignition unit of an afterburner associated with a turbojet engine. The device controls the injection time and/or the ignition fuel to the ignition unit of an afterburner associated with a turbojet engine. The device controls the injection time and/or the ignition fuel quantity as a function of the varying operating parameters of a turbojet engine under various ambient conditions. The device comprises pressure relief valves arranged and designed so that they ensure essentially the said rapid light-off or ignition of the afterburner under any operating condition as a function of the pressure of the high pressure fuel increasing or decreasing with higher or lower flight altitudes.
The McCombs, Jr. patent describes an afterburner fuel manifold flow sensor and igniter control apparatus. The apparatus comprises a flow restricting means in series flow relationship with the fuel supply conduit, a variable volume chamber and a metering restriction. Fluid pressure differential across the metering restriction is controlled by a varying device. In operation, the apparatus senses the rate of fuel flow into an afterburner fuel manifold, and energizes ignition apparatus for a predetermined time interval to ignite the afterburner fuel flow when the manifold is filled to a predetermined level.